Pressurized-water nuclear reactors comprise a primary circuit, in which the cooling water of the reactor is maintained at a high pressure of the order of 155 bars by means of a pressurizer arranged on one of the branches of the primary circuit.
The pressurizer makes it possible to keep the pressure in the primary circuit between predetermined limits, either by spraying when the pressure tends to exceed the permissible upper limit or by electrical heating of the primary fluid when the pressure tends to fall below the permissible lower limit. These operations are carried out inside the pressurizer which comprises a generally cylindrical casing, arranged with its axis vertical and having in its lower part closed and its upper part by means of domed bottoms.
The lower domed bottom has passing through it sleeves in which are introduced electrical heating pipes penetrating into the pressurizer.
The upper domed bottom has passing through it a connection piece for the spray piping of the pressurizer, connection pieces for instrumentation devices and at least one inspection port closeable by a sealing closure means.
The casing of the pressurizer of nuclear reactors being built at the present time has a diameter of the order of 2.40 m and a height of the order of 13 m.
During shutdowns of the nuclear reactor for the refuelling and maintenance of the reactor, various checking and repair operations are conducted on the pressurizer.
In particular, a check of the heating pipes and replacement of defective pipes penetrating into the pressurizer via its lower bottom are carried out. These operations can be conducted from outside the pressurizer by using at least partially automated devices.
The operators work in a zone adjacent to the outer wall of the casing of the pressurizer for a relatively short period of time. These operators are therefore subjected to low radiation doses.
In contrast, where work in the upper part of the pressurizer is concerned, for example for repairing or replacing various elements, such as the instrumentation connection pieces, it is necessary for the operators to enter the casing of the pressurizer, i.e., a zone which is highly activated because the primary fluid in the reactor comes in contact with the inner surface of the casing of the pressurizer during the operation of the reactor. Moreover, the maintenance or repair operations which have to be carried out in the upper part of the pressurizer make it necessary to install work platforms or floors inside the pressurizer because of the considerable height of the casing. These platforms or floors rest on the upper part of scaffolding which bears on the bottom and on the side wall of the pressurizer.
The scaffolding necessary for working in the upper part of the casing of the pressurizer have a height of approximately 10 m and consist of elementary parts which can be joined together, for example by snapping.
The mounting of such scaffolding of great height inside the casing of the pressurizer is an operation which is difficult and tricky to carry out and which requires a relatively long period of execution.
The operators who have to work in a highly contaminated zone where there is considerable radiation are liable to receive high radiation doses. In some cases, the radiation in particular parts of the pressurizer can be so great that it is impossible to consider carrying out an operation to mount a work platform.
Furthermore, insofar as the scaffolding consists of separate parts which can be snapped together, some of these parts can accidentally fall to the bottom of the pressurizer during the mounting operation and damage the end parts of the heating pipes projecting inside the casing of the pressurizer.